DE PS 30 48 649 describes a process for the recovery of halogenated hydrocarbons adsorbed in activated carbon. In this process, hot air flows through the activated carbon, whereby the adsorbed hydrocarbons are desorbed and pass over into the air flowing through. The air flowing off is then cool ed, which results in a condensation of the hydrocarbons dissolved therein. By again heating and returning the air to the activated carbon, this procedure can be repeated several times. However, strict attention must always be paid to the fact that the temperature of the air remain below the decomposition temperature of the halogenated hydrocarbons. The upper limit for the temperature of the hot air flowing through is, for example, 120.degree. C. with trichloroethylene and 150.degree. C. with perchloroethylene. These values apply to normal atmospheric surroundings. Atmospheric pressure of about 1000 mbar can thereby be considered to be normal atmospheric pressure.
The disadvantage of this process is that, in spite of repeating the procedure several times, there is no complete desorption of the hydrocarbons from the activated carbon filter. In addition, the air has a residue of hydrocarbons after the process has been completed.
Other embodiments of this process are published in the Patent Abstracts of Japan, Unexamined Applications, Section C, Volume 2, No. 59, 27. April 1978, page 458 C 78, No. 53-18504 and in EP 0 381 942. In these publications, the desorption of the adsorbed hydrocarbons is promoted by applying a negative pressure to the adsorber chamber. In EP 0 381 942, the adsorber is first heated to a temperature which is below the decomposition temperature of the adsorbed substance, the adsorber chamber is then sealed off from the surroundings and a high negative pressure is applied. Finally, the desorbed substance is drawn off from the adsorber chamber and the temperature and pressure ratios set in such a way that it condenses.
This process, which is promoted by negative pressure, is in fact superior to the first process mentioned. However, it also does not enable a complete desorption since the maximum working temperature is again limited by the decomposition temperature of the adsorbed substance. Using higher temperatures, at which a substantially improved desorption would take place, is thus not possible.